.\" $OpenBSD: X509_check_host.3,v 1.6 2020/09/17 08:04:22 schwarze Exp $
.\" full merge up to: OpenSSL a09e4d24 Jun 12 01:56:31 2014 -0400
.\" selective merge up to: OpenSSL 6328d367 Jul 4 21:58:30 2020 +0200
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.\" Viktor Dukhovni <openssl-users@dukhovni.org>.
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.Dd $Mdocdate: September 17 2020 $
.Dt X509_CHECK_HOST 3
.Os
.Sh NAME
.Nm X509_check_host ,
.Nm X509_check_email ,
.Nm X509_check_ip ,
.Nm X509_check_ip_asc
.Nd X.509 certificate matching
.Sh SYNOPSIS
.In openssl/x509v3.h
.Ft int
.Fo X509_check_host
.Fa "X509 *x"
.Fa "const char *name"
.Fa "size_t namelen"
.Fa "unsigned int flags"
.Fa "char **peername"
.Fc
.Ft int
.Fo X509_check_email
.Fa "X509 *x"
.Fa "const char *address"
.Fa "size_t addresslen"
.Fa "unsigned int flags"
.Fc
.Ft int
.Fo X509_check_ip
.Fa "X509 *x"
.Fa "const unsigned char *address"
.Fa "size_t addresslen"
.Fa "unsigned int flags"
.Fc
.Ft int
.Fo X509_check_ip_asc
.Fa "X509 *x"
.Fa "const char *address"
.Fa "unsigned int flags"
.Fc
.Sh DESCRIPTION
The certificate matching functions are used to check whether a
certificate matches a given hostname, email address, or IP address.
The validity of the certificate and its trust level has to be checked by
other means.
.Pp
.Fn X509_check_host
checks if the certificate Subject Alternative Name (SAN) or Subject
CommonName (CN) matches the specified hostname, which must be encoded
in the preferred name syntax described in section 3.5 of RFC 1034.
By default, wildcards are supported and they match only in the
left-most label; they may match part of that label with an
explicit prefix or suffix.
For example, by default, the host
.Fa name
.Qq www.example.com
would match a certificate with a SAN or CN value of
.Qq *.example.com ,
.Qq w*.example.com
or
.Qq *w.example.com .
.Pp
Per section 6.4.2 of RFC 6125,
.Fa name
values representing international domain names must be given in A-label
form.
The
.Fa namelen
argument must be the number of characters in the name string or zero, in
which case the length is calculated with
.Fn strlen name .
When
.Fa name
starts with a dot (e.g.\&
.Qq .example.com ) ,
it will be matched by a certificate valid for any sub-domain of
.Fa name ;
see also
.Fa X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS
below.
.Pp
When the certificate is matched and
.Fa peername
is not
.Dv NULL ,
a pointer to a copy of the matching SAN or CN from the peer
certificate is stored at the address passed in
.Fa peername .
The application is responsible for freeing the peername via
.Xr free 3
when it is no longer needed.
.Pp
.Fn X509_check_email
checks if the certificate matches the specified email
.Fa address .
Only the mailbox syntax of RFC 822 is supported.
Comments are not allowed,
and no attempt is made to normalize quoted characters.
The
.Fa addresslen
argument must be the number of characters in the address string or zero,
in which case the length is calculated with
.Fn strlen address .
.Pp
.Fn X509_check_ip
checks if the certificate matches a specified IPv4 or IPv6 address.
The
.Fa address
array is in binary format, in network byte order.
The length is either 4 (IPv4) or 16 (IPv6).
Only explicitly marked addresses in the certificates are considered;
IP addresses stored in DNS names and Common Names are ignored.
.Pp
.Fn X509_check_ip_asc
is similar, except that the NUL-terminated string
.Fa address
is first converted to the internal representation.
.Pp
The
.Fa flags
argument is usually 0, but it can be the bitwise OR of the following
flags.
.Pp
The
.Dv X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT
flag causes the function to consider the subject DN even if the
certificate contains at least one subject alternative name of the right
type (DNS name or email address as appropriate); the default is to
ignore the subject DN when at least one corresponding subject
alternative names is present.
.Pp
The remaining flags are only meaningful for
.Fn X509_check_host .
.Pp
The
.Dv X509_CHECK_FLAG_NO_WILDCARDS
flag disables wildcard expansion.
.Pp
The
.Dv X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS
flag suppresses support for
.Qq *
as a wildcard pattern in labels that have a
prefix or suffix, such as
.Qq www*
or
.Qq *www .
.Pp
The
.Dv X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS
flag allows a
.Qq *
that constitutes the complete label of a DNS name (e.g.\&
.Qq *.example.com )
to match more than one label in
.Fa name .
.Pp
The
.Dv X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS
flag restricts
.Fa name
values which start with
.Qq \&. ,
that would otherwise match any sub-domain in the peer certificate,
to only match direct child sub-domains.
Thus, for instance, with this flag set a
.Fa name
of
.Qq .example.com
would match a peer certificate with a DNS name of
.Qq www.example.com ,
but would not match a peer certificate with a DNS name of
.Qq www.sub.example.com .
.Sh RETURN VALUES
The functions return 1 for a successful match, 0 for a failed match and
-1 for an internal error: typically a memory allocation failure or an
ASN.1 decoding error.
.Pp
All functions can also return -2 if the input is malformed.
For example,
.Fn X509_check_host
returns -2 if the provided
.Fa name
contains embedded NUL bytes.
.Sh SEE ALSO
.Xr SSL_set1_host 3 ,
.Xr X509_EXTENSION_new 3 ,
.Xr X509_get1_email 3 ,
.Xr X509_new 3 ,
.Xr X509_VERIFY_PARAM_set1_host 3
.Sh HISTORY
These functions first appeared in OpenSSL 1.0.2
and have been available since
.Ox 6.1 .
